As is known, as a deflecting diaphragm, single-crystal sapphire has certain unique advantages. Not only is it a single crystal that displays no mechanical hysteresis when deflected, exhibiting only elastic deformation, but is ultra-resistant to almost any chemical attack or etching. While this may be an advantage in a finished transducer, it causes significant difficulties in fabrication.
Sapphire has a modulus of about 70.times.10.sup.6 PSI as compared to that of silicon which, in the transverse direction in the surface plane &lt;100&gt; (as defined on FIG. 4A), is about 20.times.10.sup.6 PSI. As a result, for the same electrical output from a piezoresistive Wheatstone bridge silicon grown or otherwise fastened to sapphire, the flexing diaphragm must be thinner by the ratio of ##EQU1##
This means that to fabricate a pressure transducer of sapphire with the same surface stress as one of silicon, since the surface stress is proportional to ##EQU2##
where a is the radius of the deflecting portion and t is the thickness, a diaphragm of sapphire must have a much larger diameter than one of silicon, or be much thinner.
However, because of the inert nature of sapphire, it is almost impossible to thin the sapphire diaphragm by conventional means. Further, making a diaphragm of sufficient size to get enough stress lowers the number of sensors that can be made from an individual slice as well as lowering the resultant natural frequency of the finished sensors. Additionally, slices of commercially available sapphire are usually thick (about 0.020"). However, for a relatively small diameter sensor, a thickness on the order of 0.005 inches is required.
The present invention is designed to overcome these constraints and to produce a relatively smaller silicon on sapphire sensor with enhanced characteristics.